Placeholders are used heavily throughout the library. Every placeholder must derive from placeholder. The library provides a number of placeholders, out of the box, but you are welcome to define your own, if you want more descriptive names. The placeholder _self is special in that it is used as the default wherever possible.
What exactly is a placeholder? Placeholders act as a substitute for template parameters in concepts. The library automatically replaces all the placeholders used in a concept with the actual types involved when it stores an object in an any.
For example, in the following,
any<copy_constructible<_a>, _a> x(1);
When there are multiple placeholders involved, you will have to use tuple, or pass the bindings explicitly, but the substitution still works the same way.